With reference to FIG. 1, a ducted fan gas turbine engine generally indicated at 10 has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, and intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
The gas turbine engine 10 works in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 14 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
Gas turbine engines comprise a multitude of hollow axisymmetric components. Many of these components are annular. Some annular components may be connected to coaxial components by circumferentially spaced spokes extending between the components. Examples of annular components without spokes include front and rear fan casings, ducts and the nacelle 21. Examples of annular components with spokes include front frames of gas turbine engines used in military aircraft, front bearing housings, intermediate casings and tail bearing housings.
Many of the annular components either enclose a set of rotor blades, or are connected to another annular component that encloses a set of rotor blades. Thus, it is important for an annular component to maintain roundness and to have high stiffness. If the annular component deforms, for example to an oval cross section, the blade tip clearance can be too low in some regions and too high in other regions. Too low a blade tip clearance can result in excessive wear of the annular component, and too high a blade tip clearance can increase leakage over the blade tips and reduce engine efficiency.
Another hollow axisymmetric component of a gas turbine engine is the conical nose cone. This has to be strong and stiff to resist external impacts, such as bird strikes.
A given hollow axisymmetric component may be manufactured as a single piece, for example a single piece casting, or may be fabricated from multiple components joined together, for example by welding. An example of a fabricated annular component is the nacelle.
At present, many hollow axisymmetric components are made from metal. However, some components, including fan casings, containment casings, nacelles and nose cones can be manufactured from composites. Composites components can be lighter weight than the metal equivalents. Additionally, the strength and stiffness characteristics of an composite component can be optimised by e.g. controlling the orientation of the reinforcing fibres in the composite.
To manufacture a composite component, the fibres, resin and other constituents of the composite material are typically combined and compacted into a mould and then cured. The curing is an exothermic chemical process that takes place at elevated temperature and pressure. The temperature of the composite constituents during curing affects the amount of shrinkage and thence the distribution of residual stresses in the annular component. These residual stresses can relax to cause a final product to distort from the mould shape. Annular components with spokes are particularly prone to residual stress-induced distortion. Generally, the moulding and curing processes are controlled to reduce or eliminate distortion of the component. Where distortion cannot be eliminated, components may be manufactured oversize, and then finished to the required dimensions, for example by machining. The need for a substantial finishing process increases both the time and the cost of manufacture.